1. Field of the Invention
The present invention relates to the preparation of new oxidation inhibitor additives and to their use in lubricating compositions.
2. Description of the Related Art
It is known that lubricating oils, and in particular crankcase oils intended for internal combustion engines, contain various additives used to improve their performance. Some of these additives are employed to protect the oil against the oxidation caused by the high temperatures or by certain metal ions dispersed in the oil.
This degradation of the lubricant is related to the thermal and oxidative decomposition of the oil and of the additives. It gives rise to the formation and to the deposition of carbonaceous substances on the walls of the piston. If these deposits become too great, they can initiate gumming up or seizing of the piston and can increase the wear of the moving metal parts. It is therefore important to limit the degradation of the lubricants as much as possible by employing effective antioxidant additives in economically reasonable concentrations.
The protection of lubricants by means of the various organic additives is widely described. Depending on their mode of action, a distinction is generally made among these compounds between, on the one hand, free radical inhibitors (hindered phenols, aromatic amines, phenothiazines, etc.) and, on the other hand, products which decompose hydroperoxides or peroxides (metal dithiocarbamates or dithiophosphates, phosphites, thioethers, etc.).
There are also known metal deactivators which prevent the metal ions present in the oil or on the surfaces from initiating an autooxidation of the lubricant. These products act by complexing the metals.
Among all these compounds, aromatic amines and more particularly phenothiazine derivatives are the most effective, because they act according to a number of mechanisms: decomposition of the hydroperoxides, electron transfer agents, metal chelation and oxygen acceptors.
Thus, U.S. Pat. No. 2,781,318 describes the use in synthetic lubricants of alkylphenothiazines whose alkyl groups are carried by the aromatic carbons of phenothiazine.
U.S. Pat. No. 3,536,706 mentions the use of phenothiazines substituted on the aromatic nuclei by tertiary alkyl groups containing 4 to 12 carbon atoms or by aryl groups.
U.S. Pat. No. 3,803,140 suggests the use of tert-alkyl derivatives of N-alkyl- or N-alkenylphenothiazines. Nevertheless, tert-alkyl derivatives of phenothiazines which are not substituted on the nitrogen are preferred.
U.S. Pat. No. 3,344,068 describes the use of antioxidants derived from aromatic amines in ester-based synthetic lubricants. These additives may be N-alkylphenothiazines substituted by at least one alkyl group on aromatic nuclei.
U.S. Pat. No. 3,376,224 claims antioxidants derived from phenothiazine which are substituted by an alkoxymethyl group on the nitrogen atom.
WO Patent 88/02,007 describes the preparation and the use of N-alkylthioalkylphenothiazines obtained by condensation of a phenothiazine and of an alkylthioalkanol. The presence of a sulphur atom reinforces the antioxidant power of phenothiazine substituted in this manner.